It is well known that many software programs are vulnerable to attacks that breach the security of a computer system. For example, an attacker may attempt to exploit a buffer overflow vulnerability of a running program to inject code and cause a transfer of control to the injected code, in which case the injected code has the privileges of the attacked program. One attempt to preventing attacks on software programs is broadly referred to as instruction set randomization. Broadly speaking, instruction set randomization involves encrypting the program in some fashion and then decrypting it within the processor after the processor fetches the program from memory. In this way, the attacker's task of injecting instructions is made more difficult because the injected instructions must be properly encrypted (e.g., using the same encryption key and algorithm as the program under attack) in order to correctly execute. See for example, Counter Code-Injection Attacks with Instruction-Set Randomization, by Gaurav S. Kc, Angelos D. Keromytis, and Vassilis Prevelakis, CCS '03, Oct. 27-30, 2003, Washington, DC, USA, ACM 1-58113-738-9/03/0010, which describes a modified version of the bochs-x86 Pentium emulator. Others have pointed out deficiencies of the approach. See for example, Where's the FEEB? The Effectiveness of Instruction Set Randomization, by Ana Nora Sovarel, David Evans, and Nathanael Paul, http://www.cs.virginia.edu/feeb.